This project proposes to analyze the interactions of a Fibroblast Growth Regulatory Factor (FGRF) with its target cells. We have purified this FGRF from medium "conditioned" by exposure to density-inhibited fibroblasts and have characterized it in terms of the chemical composition and molecular weight. We are now in a good position to study the cellular sites of action and the biochemical mechanism of the FGRF. In addition, the differential of FGRF on normal and neoplastic cells will be compared and related to the mechanism of growth inhibition. The successful achievement of these goals would contribute to our understanding of the phenomenon of density-dependent inhibition of growth in cultured cells and establish the general biological significance of endogenous inhibitors in cellular homeostasis.